Control system for an infinitely variable change-speed gear with a torque converter with a lock-up clutch

ABSTRACT

PCT No. PCT/EP97/04047 Sec. 371 Date Jan. 7, 1999 Sec. 102(e) Date Jan. 7, 1999 PCT Filed Jul. 25, 1997 PCT Pub. No. WO98/05887 PCT Pub. Date Feb. 12, 1998A control system for a continuously variable transmission (17), especially for control of a lock-up clutch (10) of a torque converter (2). The lock-up clutch (10) is controlled according to a strategy whereby the torque converter (2) is considered as being serially mounted with the continuously variable transmission (17). Engagement and disengagement of the lock-up clutch (10) is exclusively a function of the control of the theoretical engine speed.

The invention relates to a control system for control of a lock-upclutch in a continuously variable transmission.

BACKGROUND OF THE INVENTION

Hydrodynamic torque converters in automatic transmissions chieflyperform the task of making possible a comfortable starting operationwith an increased start-up torque and to uncouple the torsionalvibrations of the engine from the drive line. With the constantlyincreasing demands for reducing both fuel consumption and gas emission,it has become necessary further to reduce the losses in the hydrodynamictorque converter. In the first place, this is achieved by optimizing thetorque converter itself and by using a (slip regulated) lock-up clutch.The latter objective is to reduce the share of the comfortable butdissipative hydrodynamic power transmission and thus to spare fuelwithout giving up a sufficient uncoupling of vibrations.

The control of the lock-up clutch essentially consists of a strategy andpressure modulation. In the strategy established is which state thetorque converter lock-up clutch assumes, by taking into consideration anoptimal consumption and comfort characteristic. At least two basicstates (open and closed) have been defined. Together with that, aslipping operation of the torque converter lock-up clutch can beimplemented.

DE-A 41 04 542 has disclosed a control system for control of a lock-Lipclutch in a continuously variable transmission. For control a ratio ofthe speeds of rotation of the input engine and of the primary pulley ofthe continuously variable transmission is formed. If this value is one,the torque converter lock-Lip clutch is closed. By comparing an actualratio between the primary and secondary pulleys of the continuouslyvariable transmission with the computed theoretical ratio, it isdetermined that the torque converter must be activated. In this case, adisengagement signal is generated to disengage the torque converterlock-up clutch. With the engagement and disengagement, time steps becomeeffective in order to ensure that the secondary pressure is adapted tothe torques to be transmitted--in accordance with the operating state ofthe torque converter lock-up clutch. The strategy takes intoconsideration the basic, functions for control of a torque converterlock-up clutch.

SUMMARY OF THE INVENTION

The problem on which the invention is based is to provide a strategy forcontrol of a lock-up clutch which is a component part of an operatingstrategy for a continuously variable transmission. A control system fora continuously variable transmission (17) of a motor vehicle having atorque converter (2) with a lock-up clutch (10), especially for controlof the lock-up clutch, for locking up the torque converter and means (31to 42) for detecting input signals which are derived from adriver-vehicle system and processed to an engagement and disengagementsignal in accordance with the detected states, characterized in thatsaid lock-up clutch (10) is controlled following a strategy in which thehydrodynamic torque converter (2) is considered as hydrodynamic variatorserially mounted with the continuously mounted with the continuouslyvariable transmission (17) and an en disengagement of the lock-up clutch(10) is exclusively function of the control of the engine theoreticalspeed of rotation (n₋₋ Mot₋₋ soll).

BRIEF DESCRIPTION OF THE DRAWING(S)

Other features essential to the invention and the advantages resultingtherefrom are to be understood from the description that follows of anembodiment with reference to the drawings. The drawings show:

FIG. 1 is a diagram of a continuously variable transmission within amotor vehicle;

FIG. 2 is a graphic representation for explaining the engagement anddisengagement conditions; and

FIG. 3 is a variogram of the continuously variable transmission.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 diagrammatically shows a continuously variable transmission: aninput unit 1, preferably an internal combustion engine, drives astart-up unit, preferably a hydrodynamic torque converter 2. Thehydrodynamic torque converter is constructed in a manner known already:a converter housing 3 is non-rotatably connected with an impeller 4. Aturbine wheel 5 is associated with the impeller 4. The arrangement iscompleted by a stator 6. Simultaneously with the impeller is driven anoil pump 7 to supply pressurized oil to the system.

The turbine wheel 5 is connected with an output shaft 8 of thehydrodynamic torque converter. Also with the turbine wheel connectednon-rotatably but axially movably (not shown in the drawing) is a piston9 of a lock-up clutch 10. On the periphery of the piston 9 is placed afriction lining 11.

To close the lock-up clutch 10, the oil space lying to the right of thepiston 9--according to the drawing--is loaded with pressurized oil froma hydraulic line 12. The piston 9 moves to the left and the frictionlining comes to abut on the torque converter housing 3. In this case, athrough drive exists from the input unit 1 to the output shaft 8.

If the frictional engagement connection should diminish, occurringbecause the oil pressure prevalent in the pressure space, lying to theright, is either lowered or, under mutual control, moved over to anotherhydraulic line 13, in the pressure space to the left of the piston 9,according to the drawing, a pressure is built up which moves the piston9 to the right in an opening direction. The lock-up clutch 10 is thenopened so that a differential speed (slip) prevails, between theimpeller 4 and the turbine wheel 5.

The hydraulic lines 12 and 13 are attached to a hydraulic control 14.

Connected downstream of the hydrodynamic converter 2 is a reversing setin the form of a planetary gear 15. With the latter, the forward orreverse drive directions is switched by adequate switching components(clutch and brake).

An output shaft 16 drives a continuously variable transmission 17(variator). The continuously variable transmission essentially comprisesa primary pulley S1 and a secondary pulley S2, which are formed fromtapered pulleys disposed in pairs and lodged between them is a beltdrive member 18. A primary cylinder 19 and a secondary cylinder 20 areattached, via hydraulic lines 21 and 22, to the hydraulic control 14.

An output shaft 23 drives, via reduction steps 24 and 25 and adifferential 26, axle half shafts 27 and 28 of driven wheels 29 of avehicle itself not shown.

The continuously variable transmission 17 is controlled by an electronictransmission control 30 (EGS). The function of the transmission controlis to adjust the ratio according to a preset operation strategy. To thisend, the processing of a multiplicity of operation parameters isrequired. For example, a sensor 31 detects the input variable: actualthrottle valve position ∝₋₋ DK. With a sensor 32 the actual engine speedn₋₋ Mot is detected. A sensor 33 detects the position of a controlmember 34 with which the driver of the vehicle communicates a power need(accelerator pedal wish FPW). A sensor 35 signals a manual drivercontact.

The enumeration of the possible operation parameters (input variables)is incomplete. The temperature of the hydraulic fluid, for example, isamong said variables. This is detected by means of a sensor 36. Forcertain driving situations additional parameters have to be processed.To detect cornering, it is convenient to utilize the cross accelerationand/or wheel speed differences. This is done with another sensor 37. Asensor 38 detects rises and drops.

The speeds of the impeller 4, of the turbine wheel 5 and of the outputshaft 8 are transmitted by sensors 39 and 40 to the electronictransmission control 30.

With the aid of another sensor 41 the speed of rotation n₁₃ S1 of theprimary pulley is monitored. Another sensor 42 delivers the speed ofrotation n₋₋ S2 of the secondary pulley. The input variables areprocessed by the electronic transmission control to output variableshaving different purposes (information for indication of system states,control signals for actuators, etc.). A few output variables abut asinput variables on the hydraulic control 14 in order to trigger theactuation of electromagnetic valves, for example, for adjustment of theprimary and secondary pulleys or also for control of the lock-up clutch.

Point of departure of the strategy for control of the lock-up clutch isthe idea of regarding the hydrodynamic torque converter as ahydrodynamic variator serially mounted with the continuously variabletransmission. Within the scope of a driving strategy, an operation pointis preset. A preferred process for determining an operation point in adynamic drive range is explained in the applicant's German patentapplication 196 00 915.4 of Jan. 12, 1996. To that extent, referencewill supplementarily be made to the statements contained there. Withinthe scope of this operation strategy, a reduction ratio iv₋₋ soll or aspeed of rotation of the primary pulley n₋₋ S1₋₋ soll is preset. To saidpresetting corresponds a presetting of the engine speed n₋₋ Mot₋₋ soll.When the lock-up clutch is opened, the engine operation point n₋₋ Mot(regulating variable) directs itself to said drive strategy. Let it besupplementarily observed that the driving range, on one hand, is definedby the driving performance characteristic line FL₋₋ characteristic lineup to the characteristic line of the smallest ratio iv₋₋ min₋₋characteristic line.

With this general standard an operation of the lock-up clutch becomespossible with the following advantages:

In the starting range, when the engine during gas emission runs into therange of the torque converter, the lock-up clutch becomes closed wherebyare prevented in an overshoot of the engine above the value n₋₋ S1₋₋soll of the speed of rotation of the primary pulley, preset by thedriving strategy, and thus the negative gradient of speed of rotation ofthe engine resulting during the closing process.

The theoretical value of the speed of rotation of the engine n₋₋ Mot₋₋soll is controlled in the whole driving range by the standard of anengine speed of rotation n₋₋ Mot. The regulating characteristic of thelock-up clutch regulator, especially its transition to the closed state(closing quality), is thus a function of the control of the enginetheoretical speed of rotation n₋₋ Mot₋₋ soll.

When the lock-up clutch is opened, a drop of the engine to coasting inthe torque converter range (undershoot) is prevented. The drop would befelt as a disturbance by the driver because the operation point standardn₋₋ S1₋₋ soll or iv₋₋ soll is abandoned by the sudden drop of the engineand the engine is substantially affected by the coasting characteristicof the torque converter.

Because the engine speed of rotation n₋₋ Mot is linked directly with thestandard of an operation point, a flow rate of the oilpump--corresponding to the n₋₋ S1₋₋ soll speed of rotation--is producedwhen braking. Also, clearly assisted is the downshift in direction toLOW, whereby a substantial problem in connection with a continuouslyvariable transmission is effectively obviated. In addition, too early anuncontrolled drop to idling speed is prevented.

In the technical implementation two cases are to be basicallydifferentiated:

Case 1:

The vehicle moves on flat ground or uphill; and

Case 2:

The vehicle moves downhill.

Supplementarily the variogram (FIG. 3) outside range 1 delimited by theiv₋₋ max--, iv₋₋ min-- and LOW characteristic line and the rangedelimited by the LOW-- and OD--guide beam (overdrive) (designated hereas range 2) must be differently treated.

Depending on said cases and ranges 1 and 2, the standard of theoperation point is implemented in different ways. The followingdefinitions apply here:

Regulating variable:

n₋₋ Mot

Control variable:

Range 1: n₋₋ Mot₋₋ soll

Range 2: n₋₋ Mot₋₋ soll corresponding to the theoretical speed ofrotation of the primary pulley n₋₋ S1₋₋ soll.

The value n₋₋ S1₋₋ Soll is generated here from the product of thetheoretical ratio iv₋₋ soll and the speed of rotation of the secondarypulley n₋₋ S2.

Correcting variable:

for the condition lock-up clutch-close: i₋₋ drwk=p₋₋ wk (correctingvariable torque converter clutch pressure corresponding to p₋₋ wk); andfor the condition lock-up clutch open: i₋₋ driv corresponding to thepressure in the primary cylinder P₋₋ S1.

Closing of the lock-up clutch:

The standard of an engine theoretical speed of rotation n₋₋ Mot₋₋ sollis generated during a throttle valve position <1.5% (not zero) in theranges 1 and 2 as follows:

Range 1:

The applied characteristic lines iv₋₋ min-- and iv₋₋ max-- are convertedby the speed of rotation of the secondary pulley n₋₋ S2 to minimumengine speeds of rotation n₋₋ Mot₋₋ min or maximum engine speeds ofrotation n₋₋ Mot₋₋ max and laid down in the electronic transmissioncontrol as characteristic lines. The ratios calculated to the left ofthe LOW guide beam n₋₋ S2 thus assume values which are higher than themechanical LOW ratio. In the actual drive range, for example, via fuzzycontrol equipment, an engine theoretical value of speed of rotation n₋₋Mot₋₋ soll is generated which serves as the control of the engine speedof rotation n₋₋ Mot. The standard of the engine theoretical speed ofrotation n₋₋ Mot₋₋ soll must result from the parked state of thevehicle. The standard of the operation point n₋₋ Mot₋₋ soll cannot havediscontinuity in the transition from the range 1 to the range 2 and ispreferably combined with one another.

Range 2:

The standard of the ratio iv₋₋ soll within the scope of the operationstrategy is converted via the speed of rotation of the primary pulleyn₋₋ S1 directly to a theoretical engine speed of rotation n₋₋ Mot₋₋ solland used as a control variable. n₋₋ Mot₋₋ soll follows out from thestationary state of the LOW characteristic line up to the intersectionpoint with the iv₋₋ min=n₋₋ Mot₋₋ min characteristic line.

In a position of the throttle valve <1.5% (zero) there apply, whendriving on flat ground or uphill, the modes of operation described aboveare for the ranges 1 and 2.

For driving downhill, the theoretical engine speed of rotation is presetaccording to the process explained above in relation to range 2. Thelock-up clutch is closed at the synchronization point. Bysynchronization point it is understood that the point at which the valueengine theoretical speed of rotation n₋₋ Mot₋₋ soll, formed from theproduct of the actual ratio and of the speed of rotation of thesecondary pulley n₋₋ S2, is higher or equal to the idling speed ofrotation of the engine. Thereby is ensuring that the engine is rigidlycoupled as early as possible with the adjustment of the continuouslyvariable transmission in order to achieve the best possible enginebraking effect.

For opening the lock-up clutch the next process applies:

When opening the lock-up clutch an overlapping gearshift takes placebetween the lock-up clutch and the continuously variable transmission.With the signal for opening the lock-up clutch, the ratio of thecontinuously variable transmission is used as a correcting variable inorder further to keep the engine on the actual operation point which sofar had been used for control of the ratio of the continuously variabletransmission. This means: the standard of the ratio iv₋₋ soll of thespeed of rotation of the primary pulley n₋₋ S1₋₋ soll becomes thetheoretical standard for the engine speed of rotation n₋₋ Mot₋₋ soll.The objective is to control, in an ideal way, the interference level ofthe torque converter by the continuously variable transmission. Here theopening of the lock-up clutch can be especially adapted in order toassist the regulator. As to the process of opening the lock-up clutch,no distinction is made regarding a dependence on the tractionalresistance.

The basic functions implemented in the electronic transmission controlhave top priority in relation to the demands on the operation state ofthe lock-up clutch independently of the operating strategy used.

The main conditions are defined, according to the engagement anddisengagement criteria, by the basic functions exclusively by theelectronic transmission control independently of the operating strategychosen. The engagement and disengagement criteria must necessarily besatisfied so that the lock-up clutch can be controlled by a strategy.The information of whether an engagement or disengagement of the lock-upclutch is desired is communicated to the basic functions of theelectronic transmission control via an adequate flag Z₋₋ WK.

Engagement criteria:

The lock-up clutch is engaged--Z₋₋ WK engaged--, when:

n₋₋ mot₋₋ soll>1 200 l/min AND

v₋₋ fzg >3 km/h OR

n₋₋ S2>150 l/min AND

C₋₋ getr>5° C. AND

brake=0 (that is, the brake is not actuated) AND

Direction of rotation of secondary pulley n₋₋ S2=positive.

Wherein v₋₋ fzg means the vehicle speed and C₋₋ getr, the transmissiontemperature.

Disengagement criteria:

For the disengagement of the lock-up clutch (open)--Z₋₋ WK=disengage,there apply:

Z₋₋ WK=disengage when n₋₋ Mot<1 200 l/min OR

Z₋₋ blr=1 (blocking wheels detected or ABS active).

The closing point for closing the lock-up clutch is established bytaking into account particular drive states. In determining the closingpoint in time, avoided must be drive states in which a closed lock-upclutch:

does not seem necessary (for example, in case of small loads or lowvehicle speeds like shunting) or

not convenient for reasons of comfort (for example, low driving speedand high ratio or city traffic at the low speed range where very oftentraction and coasting changes are to be expected).

For determining the point in time, a simple algorithm can serve in thesimplest case. The state flag Z₋₋ WK is set to engage as a function ofthe accelerator pedal wish (FPW) and of the vehicle speed (v₋₋ fzg) whenthe operation point is above a characteristic line. From FIG. 2tentatively seen is the curve of said characteristic line. At a vehiclespeed v₋₋ fzg of zero or near zero said ranges are separated from eachother by about 20% in an accelerator pedal wish. At a vehicle speed ofv₋₋ fzg>about 50 km/h, the signal of lock-up clutch engage is alwaysset.

Instead of a simple algorithm, an expanded algorithm can be used.Suitable for this in the first place is a control system according tofuzzy equipment by which the closing point in time can be determinedaccording to several parameters. Preferably, adequate here are thevariables: accelerator pedal wish, vehicle speed, ratio and start-upwish.

The point in time for opening the lock-up clutch can also be determinedaccording to a simple algorithm. Here the lock-up clutch is opened, viaa characteristic line, by which the vehicle speed and the vehicledeceleration are related to each other. Z₋₋ WKA=f(v₋₋ fzg,a)

The time for opening the lock-up clutch can also be determine within thescope of an expanded algorithm. The lock-up clutch here is openedaccording to a stopping and braking wish. The braking and stopping wish,in turn, are generated by fuzzy control equipments. An adaptation forfinding out the adequate opening time can be carrier out by field tests.

    ______________________________________                                               Reference numerals                                                     ______________________________________                                                1 input unit                                                                  2 torque converter                                                            3 torque converter housing                                                    4 impeller                                                                    5 turbine wheel                                                               6 stator                                                                      7 oil pump                                                                    8 output shaft                                                                9 piston                                                                     10 lock-up clutch                                                             11 friction lining                                                            12 hydraulic line                                                             13 hydraulic line                                                             14 hydraulic control                                                          15 planetary gear                                                             16 output shaft                                                               17 continuously variable transmission                                         18 belt drive member                                                          19 primary cylinder                                                           20 secondary cylinder                                                         21 hydraulic line                                                             22 hydraulic line                                                             23 output shaft                                                               24 reduction step                                                             25 reduction step                                                             26 differential                                                               27 axle half shaft                                                            28 axle half shaft                                                            29 wheels                                                                     30 transmission control                                                       31 sensors                                                                    32 sensors                                                                    33 sensors                                                                    34 control member                                                             35 sensors                                                                    36 sensors                                                                    37 sensors                                                                    38 sensors                                                                    39 sensors                                                                    40 sensors                                                                    41 sensors                                                                    42 sensors                                                             ______________________________________                                    

I claim:
 1. A control system for a motor vehicle, the control systembeing used in combination with a vehicle having an engine supplyingdriving power to a hydrodynamic torque converter (2), with a lockableconverter clutch (10), sequentially coupled to a continuously variabletransmission (17), and the control system controlling engagement anddisengagement of the converter clutch (10) to facilitate the supply ofdriving power through the hydrodynamic torque converter (2), the controlsystem further comprising:a plurality of sensors for receiving aplurality of input signals from the vehicle, during operation of thevehicle, to indicate vehicle driving requirements and drivingconditions; and strategy means for treating the hydrodynamic torqueconverter (2) as a hydrodynamic variator sequentially arranged with thecontinuously variable transmission (17) and for determining an enginenominal speed control (n₋₋ Mot₋₋ nom), calculated from said inputsignals from the plurality of sensors, and controlling engagement anddisengagement of the converter clutch (10) based solely as a function ofthe determined engine nominal speed control (n₋₋ Mot₋₋ nom).
 2. Thecontrol system according to claim 1, wherein the strategy means, duringcontrol of the converter clutch (10) via the engine nominal speed (n₋₋Mot₋₋ nom), distinguishes, between a first range (1) and a second range(2), the first range (1) is located outside a variogram and is limitedby applicable characteristics of a maximum variator ratio (iv₋₋ max),applicable characteristics of a minimum variator ratio (iv₋₋ min), andapplicable characteristics of a maximum mechanical variator ratio (LOW),and the second range (2) is defined by the applicable characteristics ofthe mechanically maximum variator ratio (LOW) and applicablecharacteristics of a mechanically minimum variator ratio (OD).
 3. Acontrol system for a motor vehicle, the control system being used incombination with a vehicle having an engine supplying driving power to ahydrodynamic torque converter (2), with a lockable converter clutch(10), sequentially coupled to a continuously variable transmission (17),and the control system controlling engagement and disengagement of theconverter clutch (10) to facilitate the supply of driving power throughthe hydrodynamic torque converter (2), the control system furthercomprising:a plurality of sensors for receiving a plurality of inputsignals from the vehicle, during operation of the vehicle, to indicatevehicle driving requirements and driving conditions; and strategy meansfor treating the hydrodynamic torque converter (2) as a hydrodynamicvariator sequentially arranged with the continuously variabletransmission (17) and for determining an engine nominal speed control(n₋₋ Mot₋₋ nom), calculated from said input signals from the pluralityof sensors, and controlling engagement and disengagement of theconverter clutch (10) based solely as a function of the determinedengine nominal speed control (n₋₋ Mot₋₋ nom); and the strategy meansincludes a terrain input indicative of whether the vehicle is travelingon one of level ground, uphill and downhill, and the terrain input isutilized to determine the engine nominal speed (n₋₋ Mot₋₋ nom).
 4. Thecontrol system according to claim 2, wherein an engine speed control, inthe first range (1), is a function of the engine nominal speed (n₋₋Mot₋₋ nom) and, in the second range (2), is a function of the enginenominal speed (n₋₋ Mot₋₋ nom) in accordance with a nominal speed of aprimary disk (n₋₋ S1₋₋ nom), and the nominal speed of a primary disk(n₋₋ S1₋₋ nom) is generated from a multiplication of a nominal variatorratio (iv₋₋ nom) and a rotational speed of a secondary disk (n₋₋ S2). 5.The control system according to claim 4, wherein the engine speedcontrol is such that the engine nominal speed (n₋₋ Mot₋₋ nom), for athrottle valve position >1.5%, is generated in the first range (1) asfollows:said minimum and maximum variator ratios (iv₋₋ min, iv₋₋ max)are converted, via the rotational speed of the secondary disk (n₋₋ S2),to a minimum engine speed (n₋₋ Mot₋₋ min) and a maximum engine speed(n₋₋ Mot₋₋ max), respectively, and stored as characteristics in anelectronic transmission control unit (EGS); and for a current driverange, a value is generated, as the engine nominal speed (n₋₋ Mot₋₋nom), which serves as the engine speed control.
 6. The control systemaccording to claim 4, wherein the engine speed control is such that theengine nominal speed (n₋₋ Mot₋₋ nom), in the second range (2), isgenerated as follows:a specified nominal variator ratio (iv₋₋ nom) isconverted, via the speed of the primary disk (n₋₋ S1), into a desirednominal engine speed (n₋₋ Mot₋₋ nom) and used as the engine speedcontrol.
 7. The control system according to claim 4, wherein thestrategy means controls the converter clutch (10), via engine nominalspeed control (n₋₋ Mot₋₋ nom), such that during downhill terrain travelof the vehicle, the converter clutch (10) is engaged provided that theengine nominal speed value (n₋₋ Mot₋₋ nom), generated from a product ofa current variator ratio (iv) and the speed of the secondary disk (n₋₋S2), is at least equivalent to one of an idling speed of the engine anda synchronous speed.
 8. The control system according to claim 4, whereinthe strategy means controls the converter clutch (10), via the enginenominal speed control (n₋₋ Mot₋₋ nom), such that upon the strategy meanssending a disengagement signal, for disengaging the converter clutch(10), the current variator ratio (iv) of the continuously variabletransmission (17) is employed as an actuation parameter for maintaininga current engine operating condition, with at least one of a specifiednominal variator ratio (iv₋₋ nom) and a specified nominal speed of theprimary disk (n₋₋ S1₋₋ nom) of the variator becoming a specified enginenominal speed (n₋₋ Mot₋₋ nom).
 9. The control system according to claim1, wherein the strategy means controls the converter clutch (10), viaengine nominal speed control (n₋₋ Mot₋₋ nom), such that the strategymeans generates an engagement signal, as a function of an acceleratorpedal request (FPW) and a vehicle speed (v₋₋ veh), for determiningengagement of the converter clutch (10).
 10. The control systemaccording to claim 1, wherein the strategy means controls the converterclutch (10), via the engine nominal speed control (n₋₋ Mot₋₋ nom), suchthat the strategy means generates a disengagement signal, based upon arelationship of vehicle speed (v₋₋ veh) and vehicle deceleration to oneanother, for determining disengagement of the converter clutch (10). 11.A method for controlling locking of a converter clutch (10) of a motorvehicle, the vehicle having an engine supplying driving power to ahydrodynamic torque converter (2), incorporating the lockable converterclutch (10), sequentially coupled to a continuously variabletransmission (17), and the method controlling engagement anddisengagement of the converter clutch (10) to facilitate the supply ofdriving power through the hydrodynamic torque converter (2), the methodcomprising the steps of:receiving, via a plurality of sensors, aplurality of input signals from the vehicle, during operation of thevehicle, to indicate vehicle driving requirements and drivingconditions; and treating, via a control strategy, the hydrodynamictorque converter (2) as a hydrodynamic variator sequentially arrangedwith the continuously variable transmission (17), determining an enginenominal speed control (n₋₋ Mot₋₋ nom), calculated from said inputsignals from the plurality of sensors, and controlling engagement anddisengagement of the converter clutch (10) based solely as a function ofthe determined engine nominal speed control (n₋₋ Mot₋₋ nom).